add "broken_image.png" as fallback

package.json

@@ -46,6 +46,7 @@
     "img/logo96.png",
     "img/logo192.png",
     "img/badge96.png",
+    "img/broken_image.png",
     "index-dev.html",
     "index.html",
     "manifest.json",

src/widgets/letter-tile.jsx

@@ -18,7 +18,9 @@ export default class LetterTile extends React.PureComponent {
           <i className="material-icons">person</i>;
       }
     } else if (this.props.avatar) {
-      avatar = <img className="avatar" alt="avatar" src={this.props.avatar} />;
+      // If avatar image is invalid, show a placeholder.
+      avatar = <img className="avatar" alt="avatar" src={this.props.avatar}
+        onError={(e)=>{e.target.onerror = null; e.target.src="/img/broken_image.png"}} />;
     } else {
       avatar = null;
     }

umd/index.dev.js

@@ -8077,7 +8077,11 @@ var LetterTile = function (_React$PureComponent) {
         avatar = react__WEBPACK_IMPORTED_MODULE_0___default.a.createElement("img", {
           className: "avatar",
           alt: "avatar",
-          src: this.props.avatar
+          src: this.props.avatar,
+          onError: function onError(e) {
+            e.target.onerror = null;
+            e.target.src = "/img/broken_image.png";
+          }
         });
       } else {
         avatar = null;